I clearly have no idea. I so wish we had phone and text details....but can hypothesize an scenario.
Maybe, maybe they called it quits. None of us truly know the deal they had together with the van...maybe it really was more of HIS thing. He told the body cam guys it was his idea, I am not saying I believe that, just that he said it.
They MAY have agreed to separate, maybe they both think her Yellowstone friend is going to pick her up and she has a trip home together with that gal. She stays with Yellowstone friend once home in FL while they sort out everything else. Brian goes hiking to spend time up in his own head, he comes back, she is gone, he is worried not realizing she was THAT serious! Waits around....no texts, no calls, no Gabby. He drives home having NO idea that she has been killed while he was out processing this on his own.
I can imagine that they may have co-mingled their funds and, while clearly technically illegal, it may not have occurred to him this was theft. He just wanted to go home, the trip was a bust and he lost the love of his life. Not trying to trigger outrage here, it is only a very, very remote, but possible scenario that I am putting forth.
He goes about his business at home until her parents start reaching out-maybe on the same day that the missing persons "incident" with the LE and Gabby's dad are connected to happens- and then, since he knows exactly what was supposed to happen, realizes she didn't end up where she was supposed to and all of a sudden they get word that the police are asking about a missing person and they get a lawyer because he knows they are on tape from MOAB, where Gabby was nearly charged and thrown in jail and no one trusts anyone anymore, especially seemingly innocent LE involvement.
This is JUST again a very very remote possibility....I would be thrilled to have it ruled out with something I may be missing.